This application claims the priority of Japanese patent documents No. 8-146981, filed Jun. 10, 1996; No. 8-153718, filed Jun. 14, 1996; and No. 8-209587, filed Aug. 8, 1996, the disclosures of which are expressly incorporated by reference herein.
The present invention relates to a purification apparatus for an exhaust gas which is discharged or emitted from an internal combustion engine such as an automobile, and particularly to an apparatus which includes a catalyst for purifying an exhaust gas from an internal combustion engine that is operated under a lean air-fuel ratio (a lean burn), and from an automobile which has such a lean burn internal combustion engine.
Exhaust gas discharged from an internal combustion engine such as an automobile includes carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxide (NOx) etc. which pollute the environment, adversely affect the human body, and disturb the growth and the development of plants.
Accordingly, up to now a great deal of effort has gone into reducing the amount of such pollutants by improving a combustion in the internal combustion engine, and developing a method for purifying the discharged or the emitted exhaust gas using a catalyst to obtain a steady result.
Gasoline engine vehicles frequently utilize a three component catalyst in which platinum (Pt) and rhodium (Rh) are main active components. The oxidation of HC and CO and the reduction of NOx are carried out at the same time to convert the above air pollution materials to harmless materials.
It is characteristic of a three component catalyst, that it operates effectively only for exhaust gases which are generated within a range (“window”) in the vicinity of a stoichiometric air-fuel ratio.
In the conventional technique, the air-fuel ratio fluctuates in accordance with an operation condition of the automobile. A fluctuation region is principally controlled to the vicinity of the stoichiometric air-fuel ratio, which is a ratio between A (weight of air) and F (weight of fuel), being about 14.7 in case of the gasoline. Hereinafter, in the present specification, the stoichiometric air-fuel ratio is represented by A/F=14.7, but this value varies in accordance with kinds of the fuels.
However, when the engine is operated under a lean air-fuel ratio in comparison with the stoichiometric air-fuel ratio atmosphere, the fuel consumption can be improved. Therefore, the development of a lean burn combustion technique is promoted; and recently automobiles have been developed in which the engine is combusted under the lean area having the air-fuel ratio of more than 18.
However, when a conventional three component catalyst is adopted for purification of a lean burn exhaust gas, although the oxidation purification with respect to HC and CO is performed effectively, the reduction of NOx is not.
Accordingly, to promote the application of the lean burn system for large size vehicles and to enlarge the lean burn combustion time (that is, enlarge the operation area of the lean burn system), it is necessary to develop an exhaust gas purification technique which is suitable to the lean burn system. Thus, the development of a technique for purifying HC, CO and particularly NOx where a large quantity of oxygen (O2) is included in the exhaust gas, has been promoted vigorously.
Japanese patent laid-open publication No. 61,706/1988 discloses a technique in which HC is supplied upstream of a lean burn exhaust gas. The operation of a catalyst is facilitated by lowering the oxygen (O2) concentration in the exhaust gas to a concentration area for effective functioning of the catalyst.
Japanese patent laid-open publication No. 97,630/1987, Japanese patent laid-open publication No. 106,826/1987 and Japanese patent laid-open publication No. 117,620/1987, propose a technique in which N included in the exhaust gas (after the conversion of an easily absorbable NO2 by oxidizing NO) is absorbed and removed by contact with a catalyst having NOx absorbing ability. When the absorption efficiency decreases, by stopping a passing-through of the exhaust gas accumulated NOx is reduction-removed using H2, HC included in a methane gas and a gasoline etc., and so that NOx absorbing ability of the catalyst is regenerated.
Further, WO 93/07363 and WO 93/08383 discloses an exhaust gas purification apparatus in which an NOx absorbent material arranged at an exhaust gas flow passage absorbs NOx from a lean exhaust gas, and when an oxygen concentration in the exhaust gas is lowered the NOx absorbent material discharges the absorbed NOx. The exhaust gas absorbs NOx during the lean atmosphere and the absorbed NOx is discharged by lowering O2, concentration in the exhaust gas which flows into a NOx absorbent.
However, in Japanese patent laid-open publication No. 61,708/1988, to attain a composition of the exhaust gas which corresponds to the air-fuel ratio of A/F=14.7 where the catalyst can function (O2 concentration having about 0.5%), it needs a very large quantity of HC. A use of a blow-by gas in this patent document is effective, but the blow-by gas does not have an amount which is sufficient for efficient to treat an exhaust gas during an operation of an internal combustion engine. It is possible technically to throw the fuel but it eliminates the fuel consumption gains achieved by the lean burn system.
In Japanese patent laid-open publication No. 97,630/1987, Japanese patent laid-open publication No. 106,826/1987 and Japanese patent laid-open publication No. 117,620/1987, to regenerate a NOx absorbent material a flow of exhaust gas is stopped and the gaseous reducing agent of HC etc. is contacted to NOx absorbent. Further, two NOx absorbent materials are provided and the exhaust gas flows alternately to these two NOx absorbent materials. It is therefore necessary to provide an exhaust gas change-over mechanism, which complicates the structure of the exhaust gas treatment apparatus.
In WO 93/07363 and WO 93/08383, the exhaust gas is flowed continuously to an NOx absorbent material, and the NOx in the exhaust gas is absorbed during the lean atmosphere. By lowering O2 concentration in the exhaust gas, the absorbed NOx is discharged and the NOx absorbent material is regenerated. Accordingly, since the change-over of the exhaust gas flow is unnecessary, the problem in the above stated system can dissolved. However, these systems require a material which can absorb NOx during the lean condition and can discharge NOx when O2 concentration in the exhaust gas is lowered. Since the repeated NOx absorption and discharge inevitably causes a periodic change of a crystal structure of the absorbent, it is necessary to take a careful consideration about the durability of the absorbent. Further, it is necessary to treat the discharged NOx; in the case of a large quantity of the discharged NOx it may be necessary to provide a post-treatment using a three component catalyst.